A metathesis reaction involves identical or different olefins that are reacted with each other and affords olefins having a different structure. This reaction is very advantageous because it can cope with changes in olefin demands by interconverting ethylene, propylene, butenes and the like that are produced by naphtha cracking at certain proportions.
Olefin production processes by a metathesis reaction have been improved. For example, Patent Document 1 discloses a process of producing propylene by a metathesis reaction of ethylene and 2-butene wherein the conversion is increased by using a catalyst mixture that contains a silica-supported tungsten oxide catalyst WO3/SiO2 and a magnesium oxide catalyst. Patent Document 2 discloses a process of producing propylene by a metathesis reaction of ethylene and n-butene which involves a metathesis catalyst/co-catalyst mixture and a small amount of hydrogen whereby the metathesis reaction can take place at a sufficiently high industrial reaction rate even at low temperatures.
However, the catalysts used in the metathesis reaction (hereinafter, also the metathesis catalysts) lower activity with time, though the deterioration degrees vary depending on reaction conditions, starting olefins or catalyst types. In particular, the catalytic activity is quickly lowered due to catalyst poisoning by impurities contained in starting olefins. For example, Non-Patent Document 1 describes that the catalytic activity is deteriorated over time due to catalyst poisoning by trace impurities contained in starting olefins (such as oxygen-containing compounds such as water, alcohols, ketones and ethers, and sulfur-containing compounds such as mercaptans and thiophenes) or due to coking that is deposition of heavy by-products on the catalysts.
These problems are addressed by sufficiently removing impurities from starting olefins beforehand by for example distillation, hydrogenation, extraction or adsorption. Alternatively, the catalysts are regenerated at regular intervals by passing an oxygen-containing gas through the reactor at high temperature to burn off poisonous substances or heavy deposits attached on the catalysts, thereby maintaining catalytic activity.
In particular, the metathesis catalysts are very liable to be poisoned by impurities, and quickly reduce the activity in the presence of very trace amounts of impurities. Therefore, it is necessary that starting olefins are purified thoroughly to remove impurities contained in the starting olefins. In general, adsorption is an effective purification method for removing trace amounts of impurities. In detail, an unpurified starting olefin is passed through an adsorption purification column filled with an inorganic material (an adsorbent) capable of high adsorption performance. Patent Document 3 describes that trace unidentified impurities contained in a starting olefin are removed with use of magnesium oxide, and the metathesis catalyst activity is greatly improved as a result.
Similar to the catalysts, the adsorbents also lower performance with time and thus they are regularly regenerated by burning off substances adsorbed thereto by passing an oxygen-containing gas at high temperature, or by detaching substances adsorbed thereto by passing an inert gas such as nitrogen.
However, insufficient regeneration or over-time degradation of adsorbents results in very trace amounts of impurities remaining in starting olefins. Such impurities are supplied to a metathesis reactor and poison the catalyst to drastically lower the metathesis catalytic activity. This problem could be coped with by exchanging the adsorbents more frequently or increasing the number of adsorption purification columns. These countermeasures, however, greatly increase costs.
Alternatively, a metathesis reaction step and a catalyst regeneration step may be operated at shorter cycles and the regeneration temperature may be raised to shorten the regeneration time whereby the reactions are repeatedly conducted for short periods but with high catalytic activity. However, catalysts and in particular isomerization catalysts such as magnesium oxide greatly reduce the surface area upon regeneration at high temperatures. The surface area of catalysts is a factor that determines catalytic performances, and a larger surface area provides a higher catalytic performance. Therefore, a reduced surface area of a catalyst by high temperature regeneration leads to lower catalytic activity. Thus, processes involving regeneration at high temperatures are not practical.